Telephone systems have traditionally used copper wires to carry telephone signals. Originally, a pair of wires was used to carry a single conversation, at audio frequencies. However, the capacity of a pair of wires can be increased through the use of special circuitry such as multiplexing, wherein the pair of wires carries multiple conversations. One type of multiplexing uses a digital carrier signal. The audio information is digitized and sent over the wires on the digital carrier signal. These special circuits utilize digital carrier signals with bit rates ranging between 56,000 bits to 3.1 megabits per second. Examples of standard digital carriers that have been adapted by the telephone industry are T-1 and T-2, which utilize pulse code modulation.
Special telephone circuits often must be transferred from old wires or cables to new wires or cables. This may be because the old cable carrying the special circuit has been damaged. Alternatively, system upgrades may require rerouting away from the old cable. The new cable is used to bypass the damaged or unwanted section of the old cable.
With ordinary audio frequency circuits, one prior art transfer procedure is to splice the new cable to the old cable at a first end of the new cable. Then, the other or second end of the new cable is spliced to the old cable. The new cable forms a bridge across the old section of cable lying between the splices. This old section of cable is then cut out from the circuit.
Thus, with audio frequencies, the new cable can be spliced one end at a time to the old cable. This procedure does not disrupt the telephone circuit because at audio frequencies, the new cable only looks like a small lumped capacitor, which does not interrupt telephone service on the old cable. However, at the higher frequencies used in digital carrier circuits, problems can arise if the new cable is spliced one end at a time to the old cable. This is because at high frequencies, the unterminated new cable can appear electrically to the telephone circuit as a short circuit or an open circuit at certain frequencies. This can severely distort or even interrupt telephone service on the old cable.
Prior art techniques reduce disruption of telephone service during the transfer of special circuits by attempting to connect both ends of the new cable to the old cable at the same time. This is performed by coordinating the electrical connection of the new cable at both of its ends. In Marston, U.S. Pat. No. 3,975,600 and Ray, U.S. Pat. No. 4,590,336, a master transfer unit is provided at one end of the new cable, while a slave transfer unit is provided at the other end of the new cable. In general, the master unit electrically disconnects the old cable and then electrically connects the new cable. The master unit sends a message to the slave unit to perform the same type of switching. When the slave unit receives the message, it disconnects the old cable and then connects the new cable.
In spite of the coordination efforts of the prior art, we have discovered that there is still some disruption to telephone service. One problem is due to the fact that at each unit, the old cable is first disconnected, thus breaking the telephone circuit, followed by the connection of the new cable. Reed relays are used to perform the switching. The relays exhibit a switching time that is long compared to the carrier bit rate. Thus, this switching procedure results in lost data. Furthermore, the relays exhibit contact bounce, further increasing the switching time and the amount of data loss.
Another problem is that when the new cable is connected to the old cable, it has a dc potential of zero volts. The old cable is at some non-zero dc level as a result of its carrying the special circuit. Connecting the new cable to the old cable produces a mismatch that results in charging the new cable. Charging produces transients, which can corrupt the digital data being transmitted.
Still another problem is that the data located between the two units is frequently lost during the transfer. We have discovered that the prior art switches special circuits without regard to the direction of data travel. Data typically travels in one direction on a special circuit. When the downstream unit is switched first, the data on the old cable continues to travel from the upstream unit to the downstream unit, only to be lost. The odds of the prior art switching the downstream unit first are about even. Thus, for about 50% of the transfers, the downstream unit is switched first, resulting in lost data.